Ceramic materials are finding increasing utility fuel cell applications. Although their inherent resistance to high temperature and chemically corrosive environments are well suited for such applications, there remains the problem of joining and/or sealing separate ceramic elements or joining ceramic and metal components. In the case of solid oxide fuel cells, ceramic electrolytes are useful for oxygen separation and charge transport at high temperatures. However, such electrolytes typically must be sealed to prevent the mixing of the fuel gas and oxidant gas species on either side of the electrolyte. The seal should not only be gas-tight, but is often also used to bond fuel cell components together. Thus, the seal has to be suitable for use in chemically and thermally extreme environments, and must also have thermal expansion characteristics comparable with those of the electrolyte.
Currently, the solid oxide electrolyte material is selected from variations on a few basic compositions. The most commonly chosen basic electrolyte materials are yttria stabilized zirconia, ceria, bismuth oxide and lanthanum gallate. The thermal expansion coefficient (CTE) of these materials can range from about 100×10−7 to about 150×10−7/degree Celsius, depending on the type and concentration of dopants included therein. Fuel cells are typically operated at temperatures ranging from about 700 degrees Celsius to above 1000 degrees Celsius, depending on the type and configuration of the fuel cell. Accordingly, any sealant composition must have thermal expansion characteristics similar to those of the electrolyte (or other fuel cell components) to which the sealant is applied such that a gas tight seal is maintained at temperatures ranging from the ambient to the maximum fabrication and/or operating temperature of the resultant fuel cell device. Further, it is important that the coated substrate and the sealant not have undesired and detrimental chemical interactions. Moreover, the sealant composition must also be stable at the anticipated fuel cell operating temperature (i.e., 700-1000 degrees Celsius) for extended periods of time (i.e., the desired operating life of the fuel cell, typically about 10,000 hours) in a highly chemically reducing environments.
Various solid oxide fuel cell seal compositions have been attempted and have met with varying degrees of success. Silica, boron, and phosphate base glasses and glass-ceramics have been tried. Phosphate glasses tend to volatilize phosphates that react with the fuel cell anode to form nickel phosphide and zirconiumoxyphosphate. Further, phosphate glasses tend to crystallize to form metaphosphates and/or pyrophosphates, which are not very stabile in a humidified fuel gas at fuel cell operating temperatures.
Primarily borosilicate glasses/glass ceramics have the problem of reacting with humidified hydrogen-rich atmosphere at elevated temperatures to form the gaseous species B2 (OH)2 and B2 (OH)3. Therefore, high boron seals are apt to eventually corrode in a humidified hydrogen environment (common in fuel cell operation) over time.
Silica-based glasses and glass-ceramics fare somewhat better as fuel cell sealant material, but still have drawbacks. Although silica-based glassy materials are typically more chemically stable in a fuel cell operating environment, high-silica content glasses may have coefficients of thermal expansion sufficiently mismatched with fuel cell electrolytes and components that the seals are rapidly degraded with thermal cycling. Many of the silicate-based glasses include a BaO component to give the glass the desired CTE. BaO participates in deleterious interfacial reactions with the chromium-containing interconnect materials commonly found in solid oxide fuel cell devices, producing interfacial reaction products that compromise the mechanical integrity of the seal and/or joint.
At fuel cell operating temperatures, most glasses will crystallize relatively quickly. Thus, it is important that the coefficient of thermal expansion of not only the glass but also of the eventually formed crystallized material be compatible with the solid oxide fuel cell electrolyte. Once the glass is fully crystallized, the resultant crystalline material is typically very stable over time. Further, crystallized glasses tend to exhibit increased mechanical strength at operating temperature, translating to improved seal/joint reliability.
Fuel cell technology is becoming increasingly important as the world demand for traditional hydrocarbon fuels increases and supplies of the same decrease. As the demand for fuel cells increases, so increases the demand for sealant materials with suitable thermal, chemical, and mechanical properties. There remains a need for a sealing material composition that can operate at a temperature of up to about 1000 degrees Celsius, has a thermal expansion between 80×10−7 and 130×10−7/degree Celsius, and has no detrimental chemical interactions with the fuel cell components. The present invention addresses this need.